Murder is the unlawful killing of another human person with malice aforethought, as defined in Common Law countries. Murder is generally distinguished from other forms of homicide by the elements of malice aforethought and the lack of lawful justification. All jurisdictions, ancient and modern, consider it a most serious crime and therefore impose severe penalty on its commission.

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Common law murder is defined as the:
1. unlawful
2. killing
3. of another human person
4. with a state of mind known as "malice aforethought."

The first three elements are relatively straightforward; however, the concept of "malice aforethought" is a complex one that does not necessarily mean premeditation. The following states of mind are recognized as constituting the various forms of "malice aforethought":

(i) Intent to kill;
(ii) Intent to inflict serious bodily harm short of death;
(iii) Reckless indifference to an unjustifiably high risk to human life (sometimes described as an "abandoned and malignant heart"); or
(iv) Intent to commit a dangerous felony (the "felony-murder" doctrine).

Under state of mind (i), intent to kill, the deadly weapon rule applies. Thus, if the defendant intentionally uses a deadly weapon or instrument against the victim, such use authorizes a permissive inference of intent to kill. An example of a deadly weapon or instrument is a gun, a knife, or even a car when intentionally used to strike the victim.

Under state of mind (iii), an "abandoned and malignant heart," the killing must result from defendant's conduct involving a reckless indifference to human life and a conscious disregard of an unreasonable risk of death or serious bodily injury. An example of this is a 2007 law in California where an individual could be convicted of second-degree murder if he or she kills another person while operating a motor vehicle while being under the influence of alcohol, drugs, or controlled substances.

Under state of mind (iv), the felony-murder doctrine, the felony committed must be an inherently dangerous felony, such as burglary, arson, rape, robbery or kidnapping. Importantly, the underlying felony cannot be a lesser-included offense such as assault, otherwise all criminal homicides would be murder as all criminal homicides are felonies.

Mental disorder may apply to a wide range of disorders including psychosis caused by schizophrenia and dementia, and excuse the person from the need to undergo the stress of a trial as to liability. In some jurisdictions, following the pre-trial hearing to determine the extent of the disorder, the defense of "not guilty by reason of insanity" may be used to get a not guilty verdict.[1] This defense has two elements:

§ 40.15 Mental disease or defect. In any prosecution for an offense, it is an affirmative defense that when the defendant engaged in the proscribed conduct, he lacked criminal responsibility by reason of mental disease or defect. Such lack of criminal responsibility means that at the time of such conduct, as a result of mental disease or defect, he lacked substantial capacity to know or appreciate either: 1. The nature and consequences of such conduct; or 2. That such conduct was wrong.

A person is not criminally liable who, when the act was committed, was suffering from a psychological or neuropsychological disorder which destroyed his discernment or his ability to control his actions.

A person who, at the time he acted, was suffering from a psychological or neuropsychological disorder which reduced his discernment or impeded his ability to control his actions, remains punishable; however, the court shall take this into account when it decides the penalty and determines its regime.

Those who successfully argue a defense based on a mental disorder are usually referred to mandatory clinical treatment until they are certified safe to be released back into the community, rather than prison.[2]

Acting in self-defense or in defense of another person is generally accepted as legal justifications for killing a person in situations that would otherwise have been murder. However, a self-defense killing might be considered manslaughter if the killer established control of the situation before the killing took place. In the case of self-defense it is called a justifiable homicide.[3]

For a killing to be considered murder, there normally needs to be an element of intent. For this argument to be successful the killer generally needs to demonstrate that they took precautions not to kill and that the death could not have been anticipated or was unavoidable, whatever action they took. As a general rule, manslaughter[4] constitutes reckless killing, while criminally negligent homicide is a grossly negligent killing.[5]

Murder rates vary greatly among countries and societies around the world. In the Western world, murder rates in most countries have declined significantly during the 20th century and are now between 1-4 cases per 100,000 people per year. Murder rates in Japan, Ireland and Iceland are among the lowest in the world, around 0.5; the rate of the United States is among the highest of developed countries, around 5.5 in 2004,[8] with rates in larger cities sometimes over 40 per 100,000.[9]

Murder demographics are affected by the improvement of trauma care, leading to reduced lethality of violent assaults - thus the murder rate may not necessarily indicate the overall level of social violence.[17]

Development of murder rates over time in different countries is often used by both supporters and opponents of capital punishment and gun control. Using properly filtered data, it is possible to make the case for or against either of these issues. For example, one could look at murder rates in the United States from 1950 to 2000,[18] and notice that those rates went up sharply shortly after a moratorium on death sentences was effectively imposed in the late 1960s. This fact has been used to argue that capital punishment serves as a deterrent and, as such, it is morally justified. Capital punishment opponents frequently counter that the United States has much higher murder rates than Canada and most European Union countries, although all those countries have abolished the death penalty. Gun control advocates further point out that, unlike the United States, many European countries disallow gun ownership by private citizens but Switzerland has the least restrictive firearm laws and corresponding higher gun murder deaths. Canada introduced a comprehensive Firearms Certificate program in 1977, which was followed by a sharp decline in its homicide rate (and its firearm homicide rate) however firearm homicide rates have crept back up to pre-1977 levels by 2005 even though the overall rate remains less. Overall, the global pattern is too complex and, on average, the influence of both these factors may not be significant and could be more social, economic and cultural.